man: Spell TCP header flags names. [skip ci]

man: Spell TCP header flags names. [skip ci]

In the prose use the names from respective registry [1], which says:

* "PSH", not "PUSH",
* "CWR", not "ECN CWR", and
* "ECE", not "ECN-Echo".

This does not resolve all known inconsistencies related to TCP flags,
but at least makes the problem smaller.

1: https://www.iana.org/assignments/tcp-parameters/tcp-parameters.xhtml